In a typical telephone switching system, line interface circuits are used to provide resistive direct current coupling and alternating current transformer coupling to a subscriber telephone line served by the system. The line interface circuit usually comprises a transformer having a primary winding being split into equal halves for connection to tip and ring leads of the subscriber line and a secondary winding for connection to a hybrid circuit. Energizing direct current is supplied via tip and ring feed resistors and the split primary winding for operation of a telephone instrument or the like, connected at the far end of the telephone line. The line interface circuit may also include a tertiary winding, sometimes referred to as a balance winding, similar to that disclosed by V. V. Korsky in U.S. Pat. No. 4,103,112 issued on July 25, 1978. In any case the halves of the split primary winding are connected in series by a capacitor which conducts a.c. signal currents. If it were not for the capacitor, the a.c. signal currents would otherwise have to traverse the tip and ring feed resistors. The capacitance value of the capacitor is a critical factor in defining one of the component values of the terminating impedance of the line interface circuit. Therefore, within an operating territory of any telephone company this capacitance value for any particular family of line interface circuit types is predetermined.
One problem with this circuit is that its return loss characteristic in a lower portion of the voice band is at variance with that ideally required by many of the telephony standards setting organizations. For example, in North America the so-called regional operating telephone companies usually adopt operating standards referred to as the Local Area Telecommunications Authority Switching Systems General Requirements, (LSSGR). The telephone line termination standard is simply defined in terms of passive electrical components. Particularly throughout most of North America, the a.c. terminating characteristics of a line interface circuit, being connected to terminate a telephone line, are to be that of a 900 ohm resistor connected in series with a 2.16 microfarad capacitor, while the a.c. characteristic of the line is that of 800 ohms of resistance in parallel with a series combination of 100 ohms of resistance and 0.05 microfarads of capacitance. In China, for example, the ac terminating characteristics of a line interface circuit are specified as being 0.22 microfarads of capacitance in parallel with 1 kilohm of resistance all in series with 300 ohms of resistance, while the ac characteristic of the line is represented by 0.16 microfarads of capacitance in parallel with 710 ohms of resistance, all in series with 330 ohms of resistance. In Morocco, for another example, the a.c. terminating characteristic of a line interface circuit is specified as being 600 ohms of resistance, while the a.c. characteristic of the line is represented by 0.15 microfarads of capacitance in parallel with 210 ohms of resistance all in series with 880 ohms of resistance. The characteristic determining structure in the line circuit, as described in the previously mentioned patent, is that of tip and ring transformer windings being connected in series with a direct current blocking capacitor, across the telephone line. The capacitive requirement is provided by the value of the capacitor and the resistance requirement is provided by a resistive termination of a secondary winding in the transformer.
This structure would be perfectly satisfactory if it were not for the presence of tip and ring feed resistors which function to provide a path for direct energizing current flow for operation of the telephone line. As discussed in my application for patent, Ser. No. 199,745, filed May 26, 1988 with the title "TELEPHONE LINE INTERFACE CIRCUIT", the tip and ring feed resistors are usually of a sum total resistance of about 400 ohms, but can be of greater or lesser value. This 400 ohm value strikes a compromise between reasonable length of serviceable telephone line and maximum line current in a short line. As is well known, the impedance of any capacitive element varies inversely with frequency. In the case of the line circuit, voice frequency signals of less than about 500 Hz are effectively split between two significantly conductive paths, one being the intended path provided by the capacitor and the other being the unintended path of the tip and ring feed resistors in series with the battery source.
Recently, yet another problem has been recognised. In some operating telephone companies wherein a telemetry service has been introduced. The telemetry service is intended to provide a metered measure, of a supplied commodity; water, electricity, or pay television for example, to the supplier of the commodity via the telephone system. In one such arrangement, telemetry equipment at the subscriber's premise is poled in response to a reversal of the polarity of the energizing current being applied at the line interface circuit. In such instant the accumulated charge, in the capacitor connected between the tip and ring halves of the split primary winding, causes a current surge along the telephone line. Sometimes the current surge is sufficient to induce a momentary operation of the ringer in the subscriber's telephone instrument. As telemetry is conveniently performed while the subscriber's station set is in an idle condition, it is therefore usually performed sometime during the first few hours following midnight, that is when telephones are least likely to be in use. At such times, a momentary operation of the ringer in the subscriber's telephone instrument can be annoying, particularly for those telephone subscribers who are light sleepers. A specific solution for this problem is the subject of my application for patent filed on about Mar. 15, 1989, with the title "LINE INTERFACE CIRCUIT".
It is an object of the invention to provide a line interface circuit, for a telephone line, which is economically adaptable for use in various countries having various national telephone line matching impedance standards. It is also an object of the invention to provide a line interface circuit for a telephone line, wherein the effect of tip and ring feed resistor shunting of the prescribed alternating current capacitive reactance is substantially avoided.
It is a further object of the invention to provide a line interface circuit wherein the offending accumulated charge is substantially avoided so that the incidence or intensity of false ringer response to switching of battery polarity is reduced.